ofn-erase-background

[Ofnuts]

OK, need volunteers on Windows. Try this:

• Start Gimp (need a fresh Gimp instance)
  
• Create a 800x800 image
  
• Add an alpha channel to the white background
  
• Make a rectangle selection that covers on half of the rectangle
  
• [delete]
  
• You should know have a layer which is pure white on one half andfully transparent on the other
  
• Using the pointer dialog you can verify that the transparent bit is still 255,255,255, only the alpha changed.
  
• Remove the selection
  
• Filter>Blur pixellize and use a tile as big as the image (800x800)
  
• Use the pointer dialog again. The image should be a very drak gray (I get 44,44,44) with an alpha of 127.
  

Now, do the same thing on a small image (200x200). Your result should be half-transparent white
(255,255,255,127). If you confirm this I'll file a bug...

The script doesn't work because on "big" images (actually, big selections...), pixellize is producing the wrong color so the Color-erase which uses it doesn't work properly.